Process for the surface grafting of formed bodies, in particulate also microporous membranes made from nitrogen-containing polymers

ABSTRACT

Halogen-substitutable hydrogen atoms are linked to nitrogen atoms of a polymer, with ethylenically-unsaturated monomers. By using inorganic or organic hypohalogenites and/or organic N-halogen derivatives as halogenation means, hydrogen atoms on the nitrogen atoms of the polymer membranes or polymer formed materials are replaced by halogen atoms and part of these are removed by reducing agents in the presence of ethylenically unsaturated monomers, with radical grafting of these on to the nitrogen atoms, after which the remaining halogen atoms are removed by reducing agents in the absence of monomers.

The invention relates to a process for the surface grafting of formedbodies, in particular also microporous membranes made fromnitrogen-containing polymers.

State of the Art

According to a widespread grafting process, radical groups areintroduced into the chain of the base polymer, e.g. by means ofhigh-energy radiation, on which radical groups the grafting can takeplace by means of radical chain polymerization.

The generation of the activated, e.g. radical groups which effect thestarting point of the grafting takes place in these grafting processesin an untargeted fashion, that is, the grafting does not take placeexclusively, on a certain grouping of the chain of the base polymer,such as e.g. on the nitrogen atom, but rather on all positions of thepolymer chain which can be activated by means of high-energy radiation,e.g. also on methylene groups. A more or less strong degradation ofpolymer chains takes place at the same time by means of high-energyradiation and a damaging of the mechanical strength of the base polymeroccurs as a consequence of the lowering of the degree of polymerizationoccasioned therewith. Likewise undesired side reactions inradiation-induced grafting are cross-linking reactions, which result inan embrittlement of the material.

Other processes of graft copolymerization are based on activation underradical formation by means of strong oxidizing agents such as e.g.Ce^(IV) salts. This process can only be used in the case of very lowpH'es, namely below pH 2, because otherwise a hydrolytic precipitationof the Ce^(IV) salt occurs. The use of this process in the case of thepolymers intended for the process of the invention results in ahydrolytic damaging of these polymers and a diminution of the mechanicalstrength due to a reduction in the degree of polymerization can also beobserved. In addition, no purposeful grafting for increasing thechemical resistance of the base polymer is possible even according tothis process.

Other processes for graft polymerization are based on chain transfer inthat a homopolymerization of the monomer is induced by a radicalinitiator in the presence of the base polymer to be grafted, for whichhigh temperatures must generally be employed (70°-80° C.). The graftingtakes place in this instance by means of interaction of the growingpolymer radical with the base polymer. The grafting site can also not beinfluenced in a purposeful manner in this instance A furtherdisadvantage of this grafting process resides in the fact that only aslight portion of the monomer used is consumed for the grafting whereasat the same time a considerable amount of homopolymer is produced Theformation of homopolymer is undesired because this increases the amountof the required monomer, which adversely affects the economy of theprocess and, moreover, a specific method step for removing thehomopolymer becomes necessary.

The previously named grafting processes have the fact in common that thegrafting does not take place at any sharply defined position of the basepolymer, especially not with preference on the nitrogen atom. Thechemical nature of the corresponding bonds such as the peptide group andof the carbamic acid group is therefore not changed and a positiveinfluence on the chemical stability of these bonds can therefore nottake place.

It is also known that polyamides can be grafted with acrylamide oracrylonitrile in such a manner that in a first method step the hydrogenatoms on the nitrogen atoms are replaced by chlorine atoms. Thehalogen-substituted polyamides are then converted back into the initialpolymer by means of hydrazine or iron-II-salts, during which time aradical transitional state of nitrogen appears. A radical chainpolymerization on the nitrogen takes place in the presence of the namedmonomers as well as in the case of other redox-initiated polymerizationprocesses. This reaction is described e.g. by K. V. Phung and R. C.Schulz in "Makromolekulare Chemie", 180, 1825 (1979). It was used inthis paper to demonstrate the mentioned radical transitional stateduring the reduction.

Another paper which concerns grafting onto N-halogenated polyamidesdescribes the initiation by means of metal carbonyls (C. H. Bamford, F.C. Duncan, R. J. W. Reynolds in "J. Poly. Sci." part C, pp. 419-432(1968).

However, it is not economically possible with any of the known processesto carry out a surface grafting on form bodies of nitrogen-containingpolymers while controlling the penetration depth and the graftingdensity.

Problem definition

The invention has the problem of creating a process for the purposefulchanging of the surface properties of form bodies by means of surfacegrafting which is suitable both for the surface grafting of compact formbodies as well as for microporous or fibrous form bodies, in particularthose in areal web form and which does not require auxiliary deviceswhich are difficult to manage technically such as sources of high-energyradiation or the use of high temperatures.

The changes of the surface properties consist both in the increasing ofthe chemical resistance of the surface area, which should occur in everycase, as well as in the influencing of other chemical and/or physicalsurface properties, especially of the wetting- and adsorption behaviorwithout the disadvantages which occur in traditional grafting processesbecoming active. These disadvantages are, explicitly expressed: Chaindegradation and cross-linking reactions on the base polymer and/or ahigh amount of homopolymerizate as well as an unintended and/oruncontrollable progression of the grafting deep into the surface.

The invention therefore has the further problem of creating a processwhich makes it possible in the case of form bodies which are not verycompact, especially microporous membranes with a surface/mass ratio in arange of up to 50 m² /g, to carry out the grafting selectively over theentire polymer matrix or to limit it to the externally located chainareas of the base polymer.

Although it is generally less important in the case of compact formbodies whether, in addition to the pure surface grafting, a grafting ofdeeper layers of the base polymer also takes place, it is also a problemof the invention to regulate this penetration depth of the grafting evenin the case of compact form bodies because this can be significant inspecial instances. This is e.g. the case if the base polymer isconverted by means of the grafting over the entire chain length into asoluble graft copolymerizate, so that the grafted surface layer would beremoved upon contact with an appropriate solvent.

To the extent that it is a problem of the invention to create a processfor increasing the chemical stability of the base polymer, especially inthe surface area of form bodies, an increase of the resistance to chaindegradation by means of oxidative and hydrolytic degradation as well asby radiation damage is to be understood thereunder. In particular, theproblem of the process of the invention consists in converting the mostlabile groupings in the main chain of the base polymers such as thepeptide- or carbamic acid group in the surface area into a form which isless susceptible to chemical attack in order to avoid a chaindegradation and the associated loss of the typical properties of thebase polymer. A stabilization of the surface layer should also protectthe non-stabilized areas of the base polymer located thereunder fromchemical attack. As a result thereof, it is not necessary to stabilizethe entire base polymer of the form body, so that its mechanicalproperties are not altered in this area. An essential part of theproblem definition is the fact that the above-named stabilizationeffects are achieved without the use of stabilizers which can beextracted with solvent.

A further problem of the invention is a process for the production ofcomposite bodies in which the form body used for grafting is provided onthe surface with a layer of the graft polymer which is chemicallyconnected to the form body and is essentially free of individual chainsof the base polymer, so that the swelling properties of the layer of thegraft polymer are exclusively determined by the type of the monomer usedfor the grafting and thus differ in a characteristic manner from thoseof the base polymer. In particular, graft polymers are to be understoodthereunder which exhibit a high swelling capacity in aqueous media sothat they are accessible, when they are provided with chemical groupswhich enable them to reversibly or irreversibly bond certain targetsubstances, to these chemical groups for the particular targetsubstances even in the interior of this layer. The target substances canbe e.g. proteins, the groups capable of reversibly bonding can be ionicgroups or affinity ligands. The groups capable of irreversibly bondingcan be groups which can enter chemical bonds with amino- or sulfhydrylgroups of proteins under mild conditions and are known in the state ofthe art. As a result of the fact that not only the surface but also theinterior of the grafted polymer layer is accessible for the targetsubstances, an especially high bonding capacity of the compositematerial should be achieved.

Whereas the areas of application for the abovenamed composite materialsare in the area of the adsorptive separation of substances, a furtherproblem definition of the invention concerns the textile sector. Thegoal in the production of the composite bodies is here to provide theformed bodies, especially textile fibers, with a grafted polymer layerwhich differs as regards the dyeing technology from the base polymer inthe desired manner. An example for such an instance of application ispresent when the grafted-on polymer layer is to be dyed by means of aclass of dyes for which the base polymer exhibits either no or only aslight affinity. This is especially desirable from the standpoint oftextile technology when mixed fibers are to be dyed in one work step,e.g. mixed fibers of polyamides and cotton in a dye bath with reactivedyes for cellulose.

A further problem of the invention is to create a process for thepurposeful changing of the wetting properties of form bodies, especiallyin the direction of an increase in the water wettability as well as thewettability by liquid with an even greater surface tension than that ofwater such as e.g. electrolytic solutions in high concentration. Thisgoal is significant in all previously named areas of application. Inaddition to an increasing of the water wettability, there is also theproblem of creating a process for the reduction of the adsorptioncapacity for lipophilic substances. In the case of microporous membranesprimarily but not exclusively the protein adsorption should be reducedand in the case of textile fibers the contamination by fatty substances.In both cases a consequence of the reduction of the affinity forlipophilic substances is the fact that when such an adsorption hasnevertheless taken place, it can be readily be undone again. In the caseof textile fibers, this is expressed by the fact that washing can takeplace under considerably milder conditions than without such amodification of the surface. Likewise, filter membranes can be washedfree again more easily after clogging by means of such a modification.

Another problem of the invention is to make possible a process for theinfluencing of the zeta potential either in the direction of a negativeor of a positive potential. The zeta potential also has considerableinfluence both in the case of filter materials and in the case oftextiles on the properties of use as it determines the contaminationproperties in accordance with the contacting media. The electrostaticcharge of the form bodies is also closely associated with the zetapotential, which charge should also be reduced by the process of theinvention in that the surface conductivity is increased by means of theintroduction of ionic groups into the surface.

A further problem of the invention consists in the case of formed bodieswhich are not very compact and in the case of which the totaility of thebase polymer is located in a layer close to the surface in convertingthis form body entirely into a graft copolymer, during which anisotropic growth of this form body occurs with retention of its originalform and the grafted form body differs in its chemical and optionallyalso mechanical properties and/or solubility properties in a desiredmanner from the initial product. In addition to a change in the chemicalstability, these property changes can consist in an elevated as well asin a reduced solubility in certain solvents. As regards the mechanicalproperties, both an increase in the mechanical strength as well as anincrease in flexibility can be achieved.

Solution of the problem

In order to regulate the penetration depth of the grafting reaction, itis of decisive importance that the conditions of halogenation and ofreduction as well as the grafting time be monitored. It was determinedthat a high degree of chlorination before the grafting in conjunctionwith a weakly negative redox potential during the grafting in shortgrafting times counteracts a deep progression of the grafting.

The invention is accordingly based on the surprising determination thatthe higher the density of N-chlorinated nitrogen is, the stronger thelimitation of the grafting on the external parts of the chains of thebase polymer.

The mechanism by which the grafting is limited in the case of highlyhalogenated surfaces to a thinner surface layer than in the case ofsurfaces with low halogenation presently escapes scientific explanation.It is assumed, however, that a high density of halogenated nitrogenatoms inhibits the actual grafting process up to a certain degree. Thisassumption is based on the fact that the initial grafting speed in thecase of low-halogenated surfaces is at first considerably higher thanthat of highly halogenated surfaces. The degrees of grafting of high-and low-halogenated surfaces do not become equal until during the courseof a rather long grafting time. However, after this time the grafting inthe case of the higher-halogenated surfaces continues and comes to astandstill much later than in the case of the low-halogenated surfaces.

It is concluded therefrom that the originally highly halogenated surfaceis first partially dehalogenated in an induction phase until the halogendensity has been reduced to the degree at which no inhibition of thegrafting takes place any more. This applies at first only to theoutermost polymer chains whereas the chains located thereunder stillexhibit a high alogen density and the deep grafting is at first stillinhibited. The gradient of the halogen concentration is not recudeduntil during the course of the further grafting time and therewith oflonger action of the reducing agent on the chains located furtherinside, so that the inhibiting halogen concentration is also droppedbelow for the latter. A consequence of this development is that thenature of the actual surface grafting is increasingly lost as thegrafting time increases in favor of an inner grafting.

The fact that a less negative redox potential acts in the direction of apure surface grafting is also in harmony with the explanation attemptsince in the case of a less reducing medium the reduction of the halogengradient takes place more slowly than under strongly reducingconditions.

For reasons which have also not been completely explained, a grafting inthe membrane matrix also takes place if an exclusive but incompletesurface halogenation has taken place by means of the further essentialmeasures of the invention to be discussed further below. A conceivablemechanism for this process could be based on the fact that thehypohalogenation or hypohalogenous acid in hydrolytic equilibrium in theaqueous medium with the nitrogen-halogenated polymer migrates into theinterior of the matrix and results there primarily in a nitrogenhalogenation. Another hypothesis is based on the fact that a halogentransfer takes place in the solid phase, that is, as a consequence ofthermal oscillations of the chain segments, a halogen exchange takesplace between halogenated and non-halogenated polymer chains.

The previous explanations refer primarily of the penetration depth ofthe grafting. The thickness of the grafted-on layer, which isessentially determined by the chain length and chain density of thegrafted-on polymer, can be controlled by the selection of the monomerconcentration and the grafting time. A high monomer concentrationincreases both the chain length and the chain density but on the otherhand a long grafting time primarily increases the latter.

The special properties of the graft polymer are essentially determinedby the selection of the particular monomer, which will be explained moreextensively in the detailed description of the invention.

Type of form bodies

The type and use of form bodies are just as manifold as the areas ofapplication of the polymers intended for the grafting of the invention.They can be compact bodies, which also includes those bodies whichexhibit a surface which is relatively small in relation to their mass.Examples for this are plates, pipes, hoses, vessels such as bottles andthe like as well as also construction components such as gears. Lesscompact form bodies, which exhibit a relatively large surface inrelation to their mass, are foils, fibers or capillaries. Fibers in athickness range of 1-100 μm have e.g. a surface/mass ratio approximatelyin a range of 0.4 to 4 m² /g. Fibrous formed bodies can be processedfurther in the form of textile fibers to tissues and likewise tonon-woven areal structures such as fleeces, which can be used e.g. forpurposes of filtration.

The process of the invention is especially suitable for formed bodieswhich exhibit an extremely large ratio of surface to mass of the polymersuch as e.g. for microporous membranes for particle- and sterilefiltration whose base polymer frequently consists of polyamides orpolysulfonamides. The surface/mass ratio can be in the case of suchporous form bodies in a range between 5 and 50 m² /g. The webs existingbetween the pores therefore exhibit only very slight wall thicknesses,with typical values being on the order of a few hundredths to a fewtenths of a micron. Microporous membranes can be divided into ultra- andmicrofiltration membranes. The former are characterized by pore sizeswhich enable them to retain macromolecules approximately in a molar massrange between 500 and 1,000,0000 daltons whereas the latter exhibitactive pore sizes in a range between approximately 0.01 and 10 μm.Microporous membranes exhibit either a continuously microporousstructure or a microporous base structure and a skin-like layer locatedon the surface which layer is designated in the technical jargon as"skin". As a consequence of the lack of micropores, this skin does notexhibit a convective but rather only a diffuse permeability for thetransport of substances and is therefore suited for separations ofsubstances on a molecular basis. Typical industrial separating methodswhich can be carried out with the last-named mebranes comprising a skinare reverse osmosis, gas separation and pervaporation.

A further group of formed bodies with high surface-mass ratio are foamsas well as microporous, vapor-permeable materials, which latter can beused as replacement material for leather. Polymers preferred in industryfor the production of such formed bodies are the polyurethanes. Foamscan be open-cell or closed-cell. In the first instance the inventionprovides for a grafting on the total, thus also on the inner surface ofthe foam and in the other instance only on the outer surface.

Type of polymers

Aliphatic polyamides such as nylon 4, nylon 6, nylon 6.6 and higheraliphatic polyamides as well as aromatic polyamides which are known e.g.under the trade name of Nomex and Kevlar can be used, for example, toproduce such formed bodies. The repeating structural unit of the peptidebond is common to the polymer class of polyamides:

    --CO--NH--

The peptide bond can occur alone or also in combination with otherrepeating structural units such as e.g. in combination with the sulfonegroup:

    --SO.sub.2 --

Polymers with peptide groups and sulfone groups are known aspolysulfonamides and can be produced by means of the polycondensation ofdiaminodiarylsulfones with aromatic dicarboxylic acid.

Another group of polymers consisting of those which can be produced forgrafting form bodies used according to the method of the invention arethe polyurethanes. The polyurethanes differ from the polyamides in thatthey exhibit the carbamic acid group

    --O--CO--NH--

instead of the peptide bond.

In distinction to the previously named polymers suitable for the processof the invention and containing nitrogen in the main chain, polymerscontaining the nitrogen in a side chain are also suitable. This caninvolve either primary or secondary amino groups:

    --NH.sub.2, --NHR

but not tertiary amino groups. Likewise, the nitrogen-containing groupsof the side chain can be primary or secondary amide groups:

    --CO--NH.sub.2, --CO--NHR

The polymers suitable for the application of the process of theinvention have the fact in common that they have a hydrogen atom on thenitrogen which hydrogen atom can be substituted by a halogen atom,especially a chlorine or bromine atom.

The polymers provided for the invention display a number of advantageswhich have resulted in their being widely used in industry. Thus, manyrepresentatives of polyamides are characterized by a high mechanicalstrength and a high softening point whereas the polyurethanes exhibitvery advantageous elastic properties. In contrast thereto, thesepolymers exhibit certain disadvantages for practical application whichcan be traced in part to the limited stability of the peptide- and thecarbamic acid group. A degradation of these groups, e.g. by means ofhydrolytic, oxidative or radiation-chemical influences results indisadvantageous changes in the properties, as a result of which the areaof application of the formed bodies produced from these polymers islimited.

Type of monomers

Simply or multiply ethylenically unsaturated monomers are suitable forthe application of the invention in as far as they exhibit a solubility,even if slight, in primarily aqueous systems. The term "primarilyaqueous systems" denotes aqueous systems which contain, aside from themonomer, either no other organic component or a water-miscible solventin a concentration which does not attain the quantity which results in atotal precipitation of sodium dithionite. In the case of acetone assolvent, the amount can be e.g. up to 40 % by weight.

Among the group of simply ethylenically unsaturated monomers, theunsaturated carboxylic acids such as acrylic and methacrylic acid aswell as their esters and amides are suitable; methacrylic acid estersand methacrylic acid amides are especially preferred representatives.The esters which can be used are in particular: Methylmethacrylate,ethylmethacrylate, hydroxyethylmethacrylate, hydroxypropylmethacrylate,dihydroxypropylmethacrylate as well as the corresponding acrylates.Further among the methacrylates: Glycidylmethacrylate,trimethylammonium-2-hydroxypropylmethacrylate,dimethylaminoethylmethacrylate, diethylaminoethylmethacrylate,diethylene glycolmethacrylate, octaethylene glycolmethacrylate,sulfopropylmethacrylate, 2-N-morpholinoethylmethacrylate. Suitablemonomers with an amide base are: Acrylamide,dimethylaminopropylmethacrylamide, methacrylamidopropyltrimethylammoniumchloride, 2-acrylamido-2-methyl-propane sulfonic acid,N-acrylamidoglycol sulfonic acid, N-morpholinopropyl-methacrylamide,methacrylamidoglycolatemethyl ether, N-hydroxyethyl-methacrylamide,N-[tris(hydroxymethyl)]-methyl-methacrylamide. Further suitable, simplyethylenically unsaturated monomers are: Vinyl acetate, N-vinylpyrrolidone, 4-vinyl pyridine, N-vinyl imidazol.

The named monomers can be used alone or in a mixture. In particular, itis possible to use simply and multiply ethylenically unsaturatedmonomers in combination, which achieves the grafting of a cross-linkedpolymer. However, multiply ethylenically unsaturated monomers can alsobe used alone.

Suitable multiply ethylenically unsaturated monomers are:Pentaerythritol dimethacrylate, glycerol dimethacrylate,tetraethyleneglycol dimethacrylate, tetraethyleneglycol diacrylate.

Graft polymerization

The term "graft polymerization" denotes a polymerization in which a sidechain is grafted onto a polymer chain of a uniform product, which sidechain consists of one or several other monomers. The properties of thegraft copolymerizate obtained such as e.g. solubility behavior, meltingpoint, water absorption, wettability, mechanical properties, adsorptionbehavior, etc. deviate more or less sharply from those of the initialpolymer as a function of the type and amount of the grafted monomer. Thegreater the amount ratio of the grafted polymer in relation to the basepolymer, the stronger the properties of the former predominate.

In addition, the properties of the graft copolymerizate are influencedby the position at which the grafting takes place on the initialpolymer.

Graft polymerizations can be carried out both in liquid phase, that is,in a melt or solution, and in a solid phase, at which time the basepolymer must generally be in a swollen form in order to make possiblethe access of the monomer to the chains of the base polymer. Theswelling can take place either by means of the monomer itself or bymeans of a further component which does not participate itself in thepolymerization. The grafting progresses in this instance from thesurface to the interior of the polymer. The case can occur thereby thatthe graft copolymer being produced is soluble in the grafting medium,which accelerates the progress of the grafting because the diffusionpaths are not lengthened during the grafting.

Surface grafting

The term "surface grafting" in the sense of the invention signifies thatthe grafting is limited to the surface area of the form body. It is aspecial aspect of the invention that a controlled surface grafting ismade possible by being able to control the depth of the grafting bymeans of special measures.

Although the grafting process of the invention is essentially a surfacemodification of the formed bodies with the penetration depth of thegrafting limited to a few hundredths of a μm, preferably to a layerthickness of less than 0.1 μm, in the case of less compact form bodies,which therefore exhibit a very high ratio of surface to mass, so thatpractically the entire base polymer is located in the surface zone, theentire base polymer can be reacted during the grafting in the extremecase, so that after the application of the process of the invention theformed body consists in a uniform manner of the graft copolymerizate.This case is designated as matrix grafting in order to distinguish itfrom a pure surface grafting.

In a pure surface grafting only the parts of the chains of the basepolymer located directly on the surface of the formed body are grafted,so that in those instances in which the graft copolymer being producedwas converted during complete grafting into a graft copolymer soluble inthe grafting medium, the non-grafted part of the polymer chain remainsin the polymer structure, so that no separation of the graft copolymerfrom the surface of the form body is possible. Therefore, those monomerscan also be used in a pure surface grafting which would result in thecase of complete grafting in soluble products. In the case of a matrixgrafting of formed bodies in which entire chains of the base polymer aregrafted, the utility of the monomers is limited to those in the case ofwhich even the graft copolymerizate being product is insoluble. This canbe achieved, if required, by means of a cross-linking grafting under theaddition of a multiple ethylenically unsaturated monomer if the monomerprovided for an application does not fulfill this precondiition itself.

The matrix grafting of non-compact form bodies is then necessary if bulkproperties such as e.g. elasticity of flexibility, the solubilityproperties or the resistance to gamma rays is to be altered. If, on theother hand, only the adsorption behavior and/or the wetting behavior areto be influenced and an alteration of the mechanical properties as wellas of the solubility are to be avoided. It is on the other handnecessary to avoid a progression of the grafting deep into the polymerof the matrix.

In the case of the matrix grafting of non-compact formed bodies, theresult of the surface grafting is thus the same as that which would havebeen obtained if the particular formed body had been produced from thestart from the graft copolymer. The advantage of subsequent grafting, incontrast thereto, resides in that fact that appropriate productionmethods are frequently known or possible for the base polymers whereasthey are either unknown or cannot be carried out in principle for thegraft copolymer. Thus, for example, suitable processes for producingmicroporous membranes of polyamides or polysulfonamides as well as forproducing open-pore foams of polyurethanes are known. However, thesetechniques can not be applied straight away to the corresponding graftcopolymerizates. If the graft copolymerizates are products which areinsoluble in common solvents, these processes can even not betransferred in principle.

On the other hand, there are instances in which a pure surface grafting,that is, a sharp delimitation between the largely unchanged base polymerand the grafted-on polymer is of decisive significance for the intendedapplication of the grafted formed bodies. Such instances are especiallypresent if the base polymer is to function as carrier for the polymergrafted on in a thick layer and if this grafted-on layer should belargely free of chains of the base polymer.

An important example for such an instance is present if a compositematerial is to be produced for adsorptive separations of substances suchas e.g. ion exchange or affinity chromatography by means of surfacegrafting onto microporous membranes. It is not only important thereby inthe interest of a high adsorption capacity that the entire inner andouter surface is coated by a layer of the grafted-on polymer but alsothat this layer exhibits a certain thickness because the entire layerthickness of the grafted-on polymer contributes to the adsorptioncapacity.

It is necessary in the case of such membranes for the adsorptiveseparation of substances that the substances to be adsorbed, e.g.proteins, can penetrate into the grafted-on polymer layer. Although itcan not be rigorously proven, it is assumed that for this the grafted-onchains must be present in a state which can be completely solvated bythe medium used and that chains of the base polymer which areoccasionally present are detrimental to this goal.

The above explanation is based on the observation that given identicalinitial membranes, the same degree of grafting and affinity ligandsintroduced in the identical manner, considerably lower bondingcapacities for the substances to be adsorbed are found if the graftingtook place over the entire layer thickness than if the grafting waslimited to the chain segments of the base polymer close to the surface.

A distinction between these two types of surface grafting, that is,between matrix grafting and pure surface grafting, is possible in thecase of microporous membranes by means of a comparison of theflowthrough characteristic and of the outer dimensions before and afterthe grafting in as far as the membranes used for the grafting are notreinforced, that is, not provided with a fleece or tissue asreinforcement material. If a matrix grafting takes place in thisinstance, an isotropic growth of the membrane matrix occurs, that is,macroscopically viewed, both a surface growth and a thickness growthoccur.

Since the pores also grow proportionally along with an isotropic growthof the membrane matrix, the number of pores per surface area dropsthereby but their size increases, so that on the whole an increase ofthe hydraulic permeability can be observed. The opposite occurs in thecase of a pure surface grafting, that is, when the grafting is limitedto the chain segments of the base polymer close to the surface. Sincethe membrane matrix itself does not change its dimensions thereby butrather only an additional layer is grafted on. the outer dimensions alsoremain constant and the grafted-on layer results in a pore constrictionso that the hydraulic permeability of the membrane decreases.

Although a distinction between these two borderline limiting forms ofsurface grafting is also conceivable polychemically, the potentialmethods for this are relatively complicated and the above-nameddistinguishing criterion appears to suffice for the practicalrequirements of membrane modification. To be sure, it can be determinedin a limiting fashion that the type of monomer used also has aninfluence thereby. If a monomer is involved whose homopolymer is solubleor sharply swellable in the medium used for the mentioned permeabilitymeasurements, then a decrease of permeability can be observed in everycase, thus also in matrix grafting. It turned out in practice thatduring the grafting of hydroxyethylmethacrylate a graft polymer isproduced which is sufficiently poorly swellable in water to make itpossible to use the above distinguishing criteria with water as mediumfor determining the hydraulic permeability.

The above explanations are not to be misunderstood in the sense that anincrease or decrease in permeability must obligatorily be measureable inthe case of the two borderline forms of surface grafting. This merelyinvolves qualitative distinguishing criteria between the two borderforms of surface grafting which are only measurable in the case ofcorrespondingly high degrees of grafting and are moreover dependent onthe pore size of the initial membranes. Thus, the lowering of thehydraulic permeability at the same degree of grafting is all the greaterthe lower the pore size of the initial membrane is. It can be indicatedas guiding principle that the named distinguishing criteria becomemeasurable in the case of membranes with a nominal pore size of 0.2 μmat degrees of grafting of above 10% by weight.

Either a high or a low degree of grafting can be striven for both inmatrix grafting and also in a pure surface grafting. Since the abovedistinguishing criteria can not be used in the case of low degrees ofgrafting, it has proven to be advantageous in these instances whenworking out the grafting conditions to work up the basic conditions atfirst at high degrees of grafting in order to assure the utility of thenamed distinguishing criteria and to subsequently adjust the desireddegree of grafting under conditions which are otherwise the same atlower monomer concentration. A non-aqueous medium can also be usedthereby if the monomer provided for the grafting results in productswhich are strongly water-swellable, so that the hydraulic permeabilityfor water decreases in every case.

On the other hand, in those instances in which a relatively thick layerof graft polymer is necessary for the application of the product to beproduced by grafting, as e.g. in the case of the production of membranesfor the adsorptive separation of substances, a reduction of thehydraulic permeability is unavoidable. so that in order to assure acertain minimum degree of hydraulic permeability of the final product, acorrespondingly coarse-pore initial membrane must be the starting point.The relation of initial porosity, degree of grafting, permeability andbonding capacity of the final product can be varied for optimization inthe manner customary with experts in the art.

It must also be considered hereby that as the pore size increases, theinner surface of a microporous membrane increases, particularly sharplyin the range up to a nominal pore size of 0.2 μm. If the proteinadsorption of the non-modified membrane is taken as a measure of theinner surface, then the relative inner surface at 0.2 μm is only 0.75,with the inner surface of a 0.1 μm membrane set equal to 1. It drops at0.45 μm to only 0.65, at 0.8 μm to 0.5 and is still 0.43 at 3.0 μm.Since the thickness of the grafted-on layer results by calculation fromthe grafted-on mass divided by the surface, given the identical degreeof grafting, the layer thickness is thus considerably greater in thecase of a coarse initial membrane than in the case of a finer initialmembrane. However, in the interest of short diffusion paths in order toachieve favorable adsorption and desorption kinetics, short diffusionpaths are generally to be preferred in membranes for adsorptiveseparations of substances. In other words, in addition to the magnitudesof bonding capacity and hydraulic permeability, the kinetic parametersmust also be included in the optimization instructions sketched abovefor the grafting of membranes for the adsorptive separation ofsubstances.

A general instruction for the production of optimum such products cannot be given because this optimum can differ greatly, depending on theapplication. If the target substance is to be extracted from a verygreat volume of a very dilute solution, high hydraulic permeabilities inconjunction with a high adsorption speed are necessary. It isadvantageous in this instance to select a fine-pore initial membrane inconjunction with a low degree of grafting. If, on the other hand, thetarget substance is present in a relatively high concentration, thebonding capacity becomes more sginificant in relation to the kineticviewpoints and more coarse-pore membranes with a high degree of graftingare to be preferred.

If only an influencing of the zeta potential or of the wettability or areduction of the non-specific adsorption is to be achieved by thesurface grafting, that is, only the surface of the graft polymer but notits volume becomes active, a pure surface grafting in conjunction withthe minimum degree of grafting necessary for a complete surface coatingshould be striven for.

The drawings are graphs as follows:

FIG. 1 is a graph showing treatment time for chlorine absorption ofnylon in chlorine bleach lye.

FIG. 2 is a graph showing treatment time in a nylon/HOCl bath.

FIG. 3 is a graph showing treatment time for chlorine absorption whereinButylhypochlorite is used.

FIG. 4 is a graph depicting treatment time for chlorine absorption inchloramine in phosphate buffer.

FIG. 5 is a graph depicting grafting extent.

FIG. 6 is a graph depicting grafting extent in another bath.

FIG. 7 is a graph depicting grafting extent in still another bath.

FIG. 8 is a graph depicting grafting extent in yet another bath.

FIG. 9 is a graph depicting grafting degree in still yet another bath.

THE HALOGENATING AGENTS

Suitable halogenating agents are inorganic and organic chloro- and bromocompounds in which the halogen occurs with the oxidation number +1.Inorganic halogenating agents are hypochlorite- and hypobromination andthe hypochlorous and hypobromous acid standing in equilibrium therewithin accordance with the pH used. t-butyl hypochlorite and organicN-chloro compounds, namely chloramine T and dichloroisocyanuric acid,can be named from among the organic chlorinating agents. The suitabilityof the halogenating agents for the process of the invention is discussedin the following in conjunction with the grafting of microporousmembranes because this involves the most critical application andconclusions which result therefrom are obvious to the expert for other,more compact formed bodies.

When using inorganic and organic hypochlorites, it is possible, given anappropriately long exposure time, to quantitatively chlorinate e.g.nylon 6.6 membranes, that is, to achieve the theoretical chlorinecontent of N-Cl nylon of approximately 24 % by weight. In contrastthereto, the surface grafting of the invention takes place already atconsiderably lower chlorine contents, approximately on the order to 5-10% of this value. A certain disadvantage of the high chlorine contentnecessary in the case of aqueous hypochlorite solutions for surfacegrafting is that on the one hand the damage to the membrane matrix dueto oxidative side reactions, especially at pH'es 6, becomes all thehigher, the higher the degree of chlorination selected, but on the otherhand only a minute fraction of the chlorine introduced is consumed forthe grafting. If the residual chlorine content remaining after thegrafting is not completely removed again by a specific reduction step,it results upon storage of the material in a total desctrution of themembrane.

The chlorine excess for the surface grafting is therefore limited in anadvantageous manner for carrying out the process of the invention to theabsolute minimum degree, which succeeds by virtue of the fact that thechlorination is preferably carried out in aqueous medium with awater-soluble, organic nitrogen halogen derivative, especially anitrogen chlorine derivative with high molar mass and as low ahydrolysis constant as possible. Typically, such nitrogen halogenderivatives are N-chloro derivatives of amides and sulfonamides likethose described in "Ullmanns Encyclopadie der Technischen Chemie"[German--"Ullmanns Encyclopedia of Technical Chemistry"], 3d edition,vol. 5 on pages 382 to 388. A typical representative of this group ofcompounds is chloramine T, the sodium salt of N-chloro-p-toluenesulfonamide with a formula weight of 230 and a hydrolysis constant ofapproximately 10⁻⁸.

A high molar weight is therefore advantageous because this reduces thediffusion in the polymer matrix and the chlorination is limited to anarea of the membrane matrix which is close to the surface. The lowhydrolysis constant is significant because as a result thereof theconcentration of the inorganic hypochlorite or of the hypochlorous acidpresent in the hydrolytic equilibrium is correspondingly low. However,the higher the concentration of hypochlorous acid, the more chlorine isalso bound in the interior of the membrane matrix.

The course in time of the chlorination of membranes with chloramine Tdiffers in a characteristic manner from that with aqueous hypochlorite.Whereas in the latter instance a constant rise of the chlorine contentcan be determined over a long time period until the theoretical contentis achieved at total chlorination, it turns out in the case ofchloramine T that even at a chlorination time on the order of one secondapproximately 50 % of the chlorine content achievable within 10 min. isabsorbed. It was surprisingly determined in longer chlorination timesthat in the case of membranes chlorinated with chloramine T, even atthese low chlorine contents the characteristics for a pure surfacegrafting were able to be determined which were not able to be achievedin the case of hypochlorite-chlorinated membranes until at magnitudes ofhigher chlorine contents.

It also turned out thereby, however, that very characteristicdifferences in the course of grafting occur in time as a function of thechlorine content and of the chlorination time. Whereas e.g. after achlorination time of 5 seconds in aqueous chloramine T solution a totalchlorine content in the membrane of 0.025 % is found, it isapproximately 0.05 % at a chlorination time of 10 min. Nevertheless, thegrafting speed, that is the weight increase during the grafting as afunction of the time in the initial phase of the grafting, isconsiderably higher in the first instance, that is, at a low chlorinecontent, than it is in the second instance. The degree of grafting ofthe higher-chlorinated membranes is not considerably higher than that ofthe lower-chlorinated membranes until the grafting process has beenextended over several hours. Thus, for example, the double chlorinecontent can result after 22 hours in a fivefold degree of grafting.

It is difficult to find an explanation for the dependency of thegrafting kinetics on the degree of chlorination; however, the effects ofthe surface grafting are clear; as already mentioned, slight surfacegrowth of non-reinforced membrane specimens as well as a great declineof the flowthrough performance function as a criterion for the surfacegrafting. It thus turns out that in the case of the low-chlorinatedmembranes a slight decline of the hydraulic permeability, if at all, anda large surface growth ca be determined from the beginning whereas inthe case of the highly chlorinated membranes a strong reduction offlowthrough and a low surface growth occur at first in relation to thedegree of grafting. After a certain period a minimum of the hydraulicpermeability is achieved which corresponds in a typical instance to onethird of the initial permeability. Thereafter, the permeability risesagain, which is accompanied by a sudden increase of the surface growth.

This can be interpreted as follows: In the case of a low halogenation,especially chlorination with a nitrogen derivative such as chloramine T,a grafting in the matrix also occurs in addition to the surfacegrafting, whereas in the case of a high chlorine content (this isrelative because in a chlorination with inorganic hypochlorite, chlorinecontents which are 1 to 2 magnitudes greater are necessary to obtainthis effect) the pure surface grafting occurs in the initial phasewhich, to be sure, is overlayed in the further course of grafting by thegrafting in the matrix. The mechanism on which the use of nitrogenhalogen derivatives with high molar mass and low hydrolysis constant ashalogenating agent is based appears to be that these halogenating agentsare only capable, due to their low diffusion coefficient in the polymermatrix, of halogenating a very thin surface layer of the polymer andthis completely if the exposure time is selected to be long enough. Thesame effects occur thereby in this thin, highly halogenated layer aswhen the entire membrane matrix is highly halogenated with inorganichypochlorites. However, since only a surface layer must be reducedthereby in the following, necessary reduction of the residual chlorinecontent, not only is this consequent step effected considerably morerapidly but only much less damaging of the membrane matrix can occur, ifat all.

Among the conditions of chlorination, the pH of the chlorination bath isof primary importance. It can be stated in general that the damaging ofthe base polymer becomes greater as the pH rises, especially at valuesabove 9. The chlorination speed also drops with rising pH. The preferredpH range is therefore between 5 and 7,with values around 6 beingespecially preferred. In the case of chloramine T, a value of 6.3appears to be especially advantageous because the water solubility ofthe product decreases sharply thereunder. The preferred concentrationrange for chloramine T is between 0.1% and 3 % with values between 1 and2 % being especially preferred.

The chlorination times with chloramine T can be between 1 sec. and 10min. with the low times, preferably 2 to 10 sec, preferred if matrixgrafting is intended. However, a low chlorination with inorqanichypochlorites is generally to be preferred for matrix grafting. In orderto achieve a pure surface grafting, the long chlorination times withchloramine T and appropriate other chlorinated organo nitrogen compoundswith a high molar weight are preferred. Typical values can be between0.5 and 10 min. and times between 1 and 5 min. are preferably used.

With inorganic hypochlorites, the preferred concentrations areapproximately 0.05 to 3 % active chlorine with the range of 0.1-0.5 %being especially preferred. The chlorination times for effecting amatrix rafting are approximately 30 sec. to 5 min., preferablyapproximately 1 to 2 min. For effecting a pure surface grafting thechlorination with inorganic hypochlorites is less preferred; however, itis rendered possible at chlorination times between 5 and 15 min.,preferably approximately 10 min.

The chlorination can also take place with organic hypochlorites inorganic solvents such as e.g. t-butylhypochlorite with methylenechloride, hexane and other inert organic solvents; the type of solventexerts a considerable influence thereby on the chlorination speed andthe attainable chlorine content. It seems that those solvents whichexhibit a swelling capacity or solvent power for the N-Cl derivativebeing produced such as e.g. chlorinated hydrocarbons and aromatichydrocarbons result in higher chlorine contents. Inversely, thechlorination times must not be extended indefinitely when using thesesolvents because the material can otherwise go into solution. Aliphatichydrocarbons, on the other hand, result in low chlorine contents and theconditions in this instance appear to be similar to those in thechlorination with chloramine T. However, this chlorination method is onthe whole less preferred, not least of all because organic hypochloritesare relatively expensive and water-sensitive substances and no advantageover e.g. chloramine T can be seen.

Organic hypobromites can also be used as an alternative to chlorinationwith inorganic hypochlorites. In a special process variant thehypobromite can be produced in situ by exposing the formed body to bebrominated successively to an atmosphere of bromine and one of ammonia.After the immediate brown coloration as a consequence of the adsorptionof elementary bromine from the gas phase, a just as spontaneousdiscoloration with formation of hypobromite takes place in theatmosphere of ammonia. This process is preferred in process variants ifa subsequent formation of homopolymerizate in the grafting bath is notproblematic, because in this instance a subsequent soaking after thehalogenation can be eliminated and the gas-phase process thus has theadvantage that halogenation is performed without liquid media and thehalogenated formed body can thus enter in a non-moistened state into thegrafting medium.

The removal of the residual halogen content from the surface of thegrafted formed body is possible in principle with a broad palette ofreducing agents in takes place most rapidly with an aqueous soluton ofsodium boron hydride, which is, however, not preferred for safetyreasons on account of the development of hydrogen occurring thereby.Other suitable reducing agents are aqueous solutions of sodium hydrogensulfite, hydrazine, iron-II-salts, rongalite at temperatures above 40°C., ascorbic acid at pH'es above 9 and sodium dithionite. The latter isthe preferred reducing agent and is used in a pH range between 5 and 9,preferably approximately pH 6-7. The concentration can be 0.1-5% byweight with a range between 1 and 2 % being preferred.

The reducing agents

The preferred reducing agent for the application of the process of theinvention is sodium dithionite as well as its related products such ase.g. rongalite. Other reducing agents such as hydrazine or ascorbicacid, the latter in the alkaline range, can also be used but are lesspreferred.

A reducing agent system which is suitable in accordance with theinvention is described in the literature in conjunction withredox-initiated polymerizations (R. W. Brown, C. V. Bawn, E. B. Hansen,L. H. Howland in "Ind. Eng. Chem." 46, pp. 1073-1080 (1954) and consistsof a combination of EDTA-complexed Fe²⁺ with rongalite at approximatelypH 10. Rongalite itself is largely ineffective at room temperature forthe process of the invention but is effective at temperatures from40°-50° C., at which it develops a sufficient redox potential.Rongalite, which is produced industrially as dithionite withformaldehyde, appears to be effective like dithionite itself in a quitesimilar manner and the addition of complexing Fe ions as well as theelevation of temperature essentially appears to bring about the releaseof dithionite.

When the process of the invention is used to influence the penetrationdepth of the grafting, not only the conditions of halogenation exert aconsiderable influence on the surface grafting, that is, the attainmentof matrix grafting or of pure surface grafting, but also the reducingagent, its concentration, the pH and the redox potential in conjunctionwith the monomer do. Note in this regard that not only the redoxpotential is important, that is, it is not immaterial with whichreducing agent a certain redox potential is adjusted or with which pH itis achieved in the case of a certain reducing agent but rather there isa dependency on each of these variables (which can not be specified asto range) which can, however, be determined by means of simple pretests.

The use of a strongly negative redox potential (high redox potential)generally tends to have the effect of favoring the matrix graftingwhereas a low redox potential pushes back the amount of matrix graftingand thus favors the pure surface grafting. In order that a grafingoccurs at all, a redox potential of at least approximately -100 mV isrequired in the case of sodium dithionite as reducing agent. Thegrafting speed at the start of the grafting increases with the redoxpotential up to approximately -400 mV. However, the higher the potentialis, the smaller the final degree of grafting in the case of longgrafting times (after several hours). The latter fact can probably beexplained in that the probability of reduction without graftingincreases at a high reduction potential.

Since the reaction speed also plays a large part in the technicalexecution of the process of the invention for the economy, the redoxpotential to be selected is an essential optimization criterion. Testseries familiar to an expert in the art can determine how high a redoxpotential should be selected without a disturbing degree of matrixgrafting occurring in the concrete instance. The potential is thereforeadvantageously selected to be as high as possible in this framework sothat the work can be performed at as high a production speed aspossible.

A pH range of 6-10 is preferred for carrying out the process of theinvention using sodium dithionite as reducing agent, with the especiallypreferred range being between 7.5 and 8.5, namely approximately 8. Theconcentration of the sodium dithionite can be between 0.02 and 1%. Thelower concentration ranges are preferred in those embodiments of theinvention in which the matrix grafting should be prevented. If agrafting bath is used thereby for a fairly long time, the redoxpotential can be determined potentiometrically during the grafting andmaintained constant by dosing in a sodium dithionite concentrate.Typical values for a suitable redox potential are between -100 and -700mV, preferably between -100 and -500 mV, especially between -200 and-300 mV. The selection of a suitable redox potential can be determinedby a few pretests. In addition, a constant pH is preferably maintainedwhich can be achieved by means of an appropriately buffered graftingmedium or likewise via the dosing in of lye.

Cause of the increased chemical resistance

In those instances in which the nitrogen atom is located in the mainchain of the base polymer, as in the polyamides and polyurethanes, anoxidative splitting of the peptide- or carbamic acid group occurs uponthe action of atmospheric oxygen under heat which splitting brings abouta diminution of the chain length and therewith a degradation of thepolymer. This reaction is accompanied by a loss of mechanical strengthas well as an embrittlement. In addition,, a brown discoloration canoccur, which is undesirable for many applications. In the same manner, achain degradation also takes place in the case of a hydrolytic attack aswell as upon the action of electromagnetic radiation, especially of UVand gamma radiation.

The elevation of the chemical resistance of the surface layer of theform bodies is surprisingly achieved by the invention in that thegrafting of the monomer takes place in a purposeful manner on thenitrogen atom of the base polymer, so that the hydrogen atoms on thenitrogen are substituted entirely or partially by the grafted-on polymerchain. The N-substituted derivatives of the base polymer produced thusrepresent totally new classes of polymers in the case of polyamides andurethanes since they no longer exhibit the typical peptide or carbamicacid group but rather secondary amide groups or N-substituted carbamicacid groups in the main chain. As a consequence of the lack of ahydrogen atom on the nitrogen and/or of the steric hindrance broughtabout by the substitution and of the associated, reduced reactivity ofthese groups, the susceptibility to hydrolysis and oxidation typical forthe initial polymers as well as the sensitivity to radiation aredecisively reduced.

Oxidation resistance

The sensitivity to oxidation of the base polymers had the result thataccording to the state of the art a stabilization takes place by meansof an addition of antioxidants if the form bodies produced therefrom areintended for use at rather high temperatures in the presence ofatmospheric oxygen. However, it can be necessary even in such instancesin which the use of the form bodies at high temperatures plays no partto treat them at high temperatures, e.g. in order to clean or sterilizethem.

Sterilization at high temperatures plays a large part in the case ofform bodies used in medicine or in the pharmaceutical or relatedindustries. Frequently used sterilization methods are based e.g. onautoclaving or on vapor-blasting with water vapor up to above 140° C.Whereas the presence of residual atmospheric oxygen generally plays alesser part during autoclaving, assuming appropriate operation of theautoclave, an exclusion of oxygen during vapor-blasting, especially inthe initial phase, can not be assured.

The use of antioxidants has serious disadvantages, even though it is aneffective measure for avoiding oxidative degradation. On the one handthe protection achieved with it is only temporary because theantioxidant is consumed by oxidation or can disappear in some othermanner from the formed body. This occurs in particular during extractionby means of liquid media which are in contact with the formed body whenit is being used. The removal of the antioxidant from the formed bodycan take place at high temperatures even via the gas phase (evaporation,sublimation) because relatively lower-molecular substances are involvedwithout exception which exhibit a certain vapor pressure.

If the antioxidant is extracted by the medium in contact with the formbody, there is not only a diminution or disappearance of the oxidationprotection but also a contamination of the medium as a consequence.Substances with a very low water solubility can be involved thereby sothat the existence of the extracted antioxidant in the medium can hardlybe demonstrated. However, in the branches of industry which arepotential candidates for using the form bodies such as e.g. thepharmaceutical or food industry, non-aqueous media such as e.g. thosebased on alcohols are also frequently used. It can be demonstrated thatform bodies such as e.g. microporous membranes stabilized in atraditional manner by antioxidants exhibit no effective stabilizationafter treatment with ethanol, which can directly demonstrate the alcoholextractability of these antioxidants.

The contamination of the media with antioxidants, which are usuallymultiple substituted phenol derivatives, must also be designated asextremely undesirable if direct side effects of these substances whicheffects are toxic or dangerous in some other manner are unknown.

It is known that even substances with an extremely low water solubilityare extracted from polymers by aqueous media if these aqueous mediacontain components which are capable of emulsification or of some otherbinding of lipoid-soluble substances. This is especially known in thecase of softeners like those used in the processing various plastics.Typical aqueous media capable of extracting lipoid-soluble substancesfrom polymers are protein solutions such as e.g. serum, blood plasma orsolutions of serum proteins like those occurring in the fractionation ofblood plasma. These media are frequently filtered for the purpose ofsterilization by means of microporous membranes from the mentionedpolymers, especially from those of polyamides, and can be contaminatedby antioxidants located therein.

In a further special instance involving textile fibers based onpolyamides a stabilization with antioxidants is out of the questionbecause the tenside-containing washing liquors, detergent solutionscustomary when washing textiles would also result in an extraction ofantioxidants, so that their effectiveness would be limited to the timeof their first usage or a new treatment with antioxidant would benecessary after each washing procedure. The oxidation sensitivity oftextile fibers of polyamides is especially noticeable due to a yellowingat high temperatures.

In the case of microporous membranes the oxidative degradation takeseffect as a rapid decline in the mechanical strength. If, for example,non-oxidation-stabilized nylon membranes are autoclaved for 1 hour at140 ° C. in the presence of atmospheric oxygen, the strength (measuredas bursting strength) drops to nearly zero. On the other hand, membranesgrafted in accordance with the invention suffer no measurable loss ofstrength in the sense of a decline of the bursting strength upon morethan one hour being autoclaved under the named conditions. Indistinction to membranes stabilized with antioxidants, this resistanceto oxidative degradation remains even if the membrane is extracted priorto being autoclaved with a customary extracting agent for antioxidantssuch as e.g. ethanol.

Hydrolysis resistance

E.g. aliphatic and aromatic polyamide to exhibit a relatively highalkali resistance; however, the hydrolyris resistance of these polymersin the acidic range must be described as slight. The hydrolysisresistance of polyurethane is unsatisfactory both in the acidic and inthe alkaline range for many applications. Polysulfonamides also exhibita low hydrolysis resistance both in an acidic and in an alkaline milieu.The hydrolytic attack on the form bodies can take place both duringtheir intended use when aqueous media with high or low pH'es and/or hightemperatures can act on them or also during vapor sterilization orautoclaving.

The hydrolytic degradation of form bodies used in the medicinal orpharmaceutical field as well as in related fields is damaging not onlyon account of the already mentioned reduction of the mechanicalstrength. There is also a risk of contanination here that hydrolysisproducts of the polymers such as e.g. hexamethylene diamine and adipicacid can pass into the medium in the case of nylon 6.6 and also ofoligomers.

The elevation of hydrolysis resistance by means of the use of theprocess of the invention is especially significant because other methodsare not industrially available for the protection of the base polymersagainst hydrolytic degradation, corresponding to the antioxidantsagainst oxidative degradation. Since the hydrolysis of form bodiesattacks the surface, just as oxidation does, a very extensive protectioncan be achieved on the surface by grafting.

Radiation resistance

The same also applies to protection against the attack of ultravioletradiation. Similar to the situation with the antioxidants, the using ofthe process of the invention renders the use of UV stabilizerssuperfluous.

The resistance to gamma radiation is of considerable industrialsignificance because radiation sterilization is widespread for formbodies used in the medical or pharmaceutical field. It is known thate.g. microporous membranes of polyamides are completely destroyed duringradiation sterilization. The elevation of the resistance of not verycompact form bodies with a high surface/mass ratio to gamma radiation issolved in accordance with the invention by means of an extensive matrixgrafting. It is thus necessary in the case of resistance to gammaradiation, on account of the greater penetration depth of thisradiation, to graft a greater amount of the base polymer than isnecessary for the elevation of the chemical and UV resistance.

Mechanical properties

A significant alteration of the mechanical properties of form bodies inaccordance with the process of the invention is limited by nature tothat group which exhibits a high surface/mass ratio because otherwise analteration of the mechanical surface properties relative to the bulkproperties does not take effect.

Base polymers such as polyamides and polyurethanes are primarily used inindustry on account of their special mechanical properties, of which inthe first instance the mechanical strength and in the second instancethe elasticity are especially exceptional. A basic alteration of thechemical nature of these polymers like that represented by thesubstitution of the hydrogen atom on the nitrogen has as its consequencea basic alteration of the intermolecular forces as the formation ofhydrogen bridges between the polymer chains is prevented, as a result ofwhich as a rule no improvement in the mechanical properties is to beexpected.

However, it is surprisingly possible according to the invention toeffect a considerable improvement of mechanical properties upon apurposeful selection of the monomers used for grafting. Thus, thegrafting of hydroxyethylacrylate onto microporous membranes of nylon 6.6and nylon 6 can eliminate the tendency of these materials towardbrittleness in a completely dry state, thus e.g. after drying at 105° C.in a drying cupboard and a totally flexible material with unimpairedmechanical strength can be obtained. If, on the other hand,hydroxyethylmethacrylate is used instead of hydroxyethylacrylate, on thecontrary, an embrittlement is more likely observed at the same degree ofgrafting.

Wettability, adsorption properties and zeta potential

The type of surface modification which can be attained is determined inan obvious manner primarily by means of the type of monomer used.Monomer mixtures can also be used if necessary if, as is explained inthe description of the industrial embodiment of the process, specialmeasures are taken which assure the reproducibility of the monomercomposition over the entire production process.

According to a preferred embodiment of the invention the grafted-onmonomers are cross-linked, preferably using bifunctional monomers, whichcross-linking takes place with advantage simultaneously with thegrafting. Unsaturated diesters of polyalcohols are mentioned as suchbifunctional monomers.

However, it can be advantageous, depending on the intended use, to leavethe grafted-on polymer non-cross-linked, in which case care should betaken that no bifunctional monomers are contained as impurities in theethylenically unsaturated monomers to be grafted.

As regards the monomer concentration, grafting time and graftingtemperature, it should be noted that if a high chain density given asmall chain length is required, e.g. as concerns the influencing ofwetting properties, adsorption properties and zeta potential, only lowmonomer concentrations and long grafting times should be used and thelowest possible degress of grafting and, correspondingly, the lowestpossible reduction of flowthrough should be striven for. If it is aquestion of grafting an initial polymer for the subsequent fixing ofligands (e.g. natural and synthetic ligands for affinitychromatography), larger chain lengths with higher degrees of graftingare to be striven for but not necessarily a high chain density. In thisinstance a higher monomer concentration is selected and shorter graftingtimes are made to suffice.

Of the monomers indicated in the listing and suitable for the graftingprocess of the invention, among which monomers the methacrylic acidderivatives are generally preferred over otherwise comparable acrylicacid derivatives on account of their considerably higher hydrolysisstability, the acrylates and methacrylates of polyalcohols such as e.g.of ethylene glycol, glycerol, diethylene glycol, octaethylene glycol andof propylene glycol are suitable both for the hydrophilizing, that is,the raising of the wettability by water and also for the diminution ofthe protein adsorption. In addition, the named monomers are suitable forproducing composite materials provided for the subsequent fixing ofaffinity ligands by means of reactions on the hydroxyl groups. Such aproduct is described in a parallel application (P 39 29 645.8-43).Hydroxyethylmethacrylate and glycerol methacrylate are especiallypreferred for this application.

The grafting of glycidyl methacrylate is preferred for the production ofbase materials for numerous further reactions which are also describedin a parallel application (ion exchangers, chelate exchangers, etc.).

For the production of ion exchangers, preferably membrane ionexchangers, the direct grafting of the ionic monomers is also apossibility, in addition to the grafting of glycidylmethacrylate and thefollowing introduction of ionic groups; however, the first-named path ismore to be preferred than the latter. The strongly acidic, weaklyacidic, strongly basic and weakly basic monomers cited in the list ofmonomers are suitable for these embodiments of the invention. If thealready mentioned measures for obtaining a low degree of grafting with ahigh chain density are used instead of a high degree of graftingnecessary for ion exchangers, the mentioned products can be obtainedwith modified zeta potential. In this connection, the requirement for alow degree of grafting is primarily significant in the case ofmicroporous membranes in order to avoid an unnecessary reduction offlowthrough.

Degree of grafting

The term "degree of grafting" denotes the increase in mass of thepolymer when using the grafting process of the invention relative to theinitial mass of the form body, expressed in % by weight. It is obviouslyunderstood that a relevant magnitude can only be present in the case ofthe non-compact form bodies because otherwise an increase in massachievable by surface grafting in a thickness range of up to 0.1 μm cannot constitute an expressive criterion.

On the other hand, in the case of non-compact form bodies, among whichthe microporous membranes constitute an extreme borderline case, thedegree of grafting can vary within very broad limits according to theprocess of the invention, according to whether only pure surfaceproperties, the adsorptive binding capacity or the bulk propertiesshould be influenced. In the sequence indicated, the ranges of thedegrees of grafting in question are approximately 1-5%, 5-45% and10-700%, which ranges are to be understood only as quite rough referencepoints.

Tempering

For reasons which have not been physically completely explained at thepresent time, the grafting behavior of the base polymers is not onlydependent on their chemical structure but also on their physicalprehistory. This will be explained as follows using the example ofpolyamides.

Polyamides, for example, appear to occur in various modifications whichdiffer in a conspicuous manner by virtue of their surface properties,especially their wetting behavior. If they are processed out of themelt, as is predominantly the case, they exhibit a high contact anglewith water, even if it is still low in comparison to most of the otherpolymers. On the other hand, if they are precipitated out of solutionsat low temperatures, as is the case e.g. in the production of membranesaccording to the method known as the "phase inversion process", thecontact angle with water is so low that spontaneous wetting occurs inthe case of microporosity. Such spontaneously wettable products areconverted by a tempering procedure near the melting point of crystalliteinto the same state which is also present in the case of a directprocessing from the melt, that is, they can then be wetted just aslittle as products prodsuced from the melt.

This phenomenon deserves to be mentioned in conjunction with the processof the invention in as far as the form bodies of nylon 6 and nylon 6.6produced by precipitation from solutions at low temperatures, whichapplies e.g. to microporous membranes of this polymer, exhibit a lesserand poorly reproducible grafting tendency. A similarly favorablegrafting behavior, both as concerns the grafting capacity as well as thereproducibility, can not be obtained, as is the case of productsobtained directly from the melt, until after a tempering step which iscarried out e.g. in superheated water vapor at temperatures between 220°and 240 ° C. The cited tempering step is therefore a preferred if notobligatory partial step when using the process of the invention onmicroporous membranes of polyamides produced according to the phaseinversion process at low temperatures.

Technical execution of the process

The industrial execution of the process of the invention is explained indetail in the following for the case of areal web materials, especiallymicroporous membranes, fleeces or tissues. The conclusions which resultstherefrom for the more compact formed bodies are not difficult for anexpert to derive.

The grafting can take place in a batch operation or in a continuousmanner. In order to achieve reproducible results over the entire weblength, it is essential that identical conditions be maintained thereby,the consistency of which conditions is subject to various requirementsin the course of time during the batch operation and the continuousprocess.

A device known in the textile industry under the designation "jigger" ora similar apparatus can be used for the batch operation. A jiggerconsists of two winding bobbins and a bath located between them. The webmaterial is wound alternatingly on the two bobbins and drawn through thebath thereby. The entire device can be hermetically closed by a hoodcover.

Since the winding speed can be selected to be very high, identicalconditions are also given for the start and the end of the web when thebath changes its composition during the course of treatment. Althoughsuch a changing is not to be striven for, it can be better toleratedthan if a continuous method is used.

This is especially significant if a mixture of monomers is grafted. As aconsequence of the different copolymerizate parameters of the individualmonomers, the grafting bath changes not only in the absoluteconcentration of monomers but also in their relationship to each other.Therefore, the grafting conditions vary in time during discontinuousgrafting but in approximately the same manner over the entire web sothat the reproducibility is assured.

The chlorination can either take place directly in the jigger or the webis introduced in a chlorinated state already. In any case, the exposuretime of the chlorination solution must be maintained in accordance withthe requirements. Thus, if a short chlorination time is intended toachieve a deep action of the grafting, then the exposure time of thechlorination bath from the immersion of the web until washing out mustbe taken into consideration, thus, in the case of a chlorination in thejigger also the dwell time in the state impregnated with thechlorination bath on the winding rollers.

A thorough soaking is to be performed after the chlorination becauseotherwise in the following grafting not only the reducing agent of thegrafting bath would be consumed in an uncontrollable manner but theadhering chlorinating agent would also form a redox system initiatinghomopolymerization with the reducing agent.

If the chlorination was carried out in the jigger, it is ready after thesoaking for the grafting and the gas chamber is washed with inert gas,for which nitrogen suggests itself, whereupon the grafting bath isfilled in under the exclusion of atmospheric oxygen. The exclusion ofatmospheric oxygen has the same reason as the washing after thechlorination, namely, the avoidance of the consumption of reducing agentand of homopolymerization.

The redox potential is preferably maintained constant in accordance withthe viewpoints of the invention by dosing in the reducing-agentconcentrate, which is preferably a 2% solution of Na dithionite, duringthe subsequent grafting, which takes place under a back-and-forthwinding of the web through the grafting bath. The pH is likewisemaintained constant by dosing lye in as far as it is not preferred toperform an appropriately strong buffering of the bath which would renderthis superfluous.

After the grafting time determined in pretests for the attainment of thedesired effect, the remaining grafting bath is removed from the jigger,a soaking performed with ample supplying of fresh water and then areduction is performed to remove the remaining chlorine content. Theconditions necessary for dechlorination depend to a large extend on thedegree of chlorination used and on the conditions of grafting and can bedetermined in pretests with the aid of iodometric chlorinatingconditions. In the normal instance, the action of a 1% Na dithionitesolution at pH 6 for 30 minutes is sufficient for dechlorination.

Both the bath method or the impregnation method are possible ascontinuous method. In the case of the bath method, the web is guided forthe required grafting time once through the grafting bath and in thecase of the impregnation method it is loaded with the impregnatingsolution and afterwards guided for the required grafting time through adwell stretch constituting an inert gas chamber. A soaking bath takesplace in both instances and then the dechlorination bath as above.

The uniformity over the web length presents certain difficulties in thefirst instance because the reproducibility is only given if the graftingconditions and therewith the composition of the grafting bath aremaintained constant for the entire grafting time. Difficulties are posedthereby in particular by maintaining the monomer concentration constantwhereas redox potential and pH, as already explained, pose few problems.The concentration of ethylenically unsaturated monomers can bedetermined via the UV adsorption with sufficient exactitude; however,this method is eliminated in the presence of Na dithionite because thereducing agent strongly absorbs in the same wavelength range. Thedensity measurement is likewise unreliable because the density of thegrafting bath varies by the necessary subsequent postaddition ofelectrolytes such as dithionite and lye.

Nevertheless, this process can be readily used in certain instances, towit, when the monomer exhibits a limited water sobulility. It ispossible in this instance, at least in the range of the saturationconcentration, to maintain a constant monomer concentration even forlong grafting times. To be sure, this is not strictly valid because aslow changing of the monomer solubility is brought about during thegrafting time by the salt concentration, which necessarily increases dueto the addition of lye and reducing agent. However, this influence canbe disregarded in a first approximation.

In contrast thereto, the influence of the electrolyte concentration onthe monomer solubility can also be utilized in a purposeful manner bypurposefully reducing the solubility of monomers, even of water-musciblemonomers such as e.g. hydroxyethylmethacrylate, by means of a highaddition of salt so that grafting can be carried out in the saturationrange with a constant concentration. However, this basically possiblepath is generally not preferred.

A few monomers which are especially suitable for the process of theinvention such as e.g. glycidylmethacrylate, vinyl acetate andmethylmethacrylate exhibit a water solubility which is within the rangesuitable for the use of the process. Thus, the water solubility of vinylacetate is approximately 1.5%, that of glycidylmethacrylateapproximately 2.5%. It is possible in these instances in a relativelyeasy manner to maintain the grafting bath continuously at the constantconcentration corresponding to the saturation, e.g. in that it issaturated with the monomer in a circulating circuit by means of anabsorption column. Another, even simpler method consists in that anemulsion of the monomer is used while employing a suitable emulsifier.The monomer represents the disperse phase in such an emulsion whereasthe continuous aqueous phase forms the grafting medium. It is easy torecognize even visually in such a process from a decrease of the milkyappearance whether the grafting medium has become depleted in monomer sothat a subsequent dosing of the emulsion can take place in a timelymanner. It is assured in any case that the monomer concentration in thegrafting medium does not drop unnoticed below the saturation.

The above-named process of emulsion grafting does have significantadvantages; however it can not be universally used in the sense of theinvention because many of the preferred monomers are miscible withwater. Moreover, it can also be advantageous to use a mixture ofmonomers. If the already described batch operation is not preferred inthese instances, e.g. because a continuous operation is to be preferredfor large production for economic reasons in every case, the alternativeis impregnation grafting using the dwell stretch in inert gas.

It should be considered thereby that neither monomer concentration,redox portential or pH can be maintained constant in time. Inversely,however, the course of these variables in time can be maintainedconstant over the entire web length with high reproducibility and anextremely high degree of constancy of the product properties can beassured therewith, even if more than one monomer is used.

Due to its universal applicability, this process variant is especiallypreferred without this modifying of the utility of the other variants inspecial instances.

The following examples exemplify the invention.

EXAMPLES 1 TO 7

Examples 1 to 7 were carried out with non-reinforced microporousmembranes with a nominal pore size of 0.2 μm and a hydraulicpermeability of 20 ml/cm² min. bar (diameter of membrane specimens 50mm).

EXAMPLE 1

Chlorination with aqueous sodium hypochlorite solution at various pH'es.

Commercially available chlorine bleaching liquour (containing sodiumhypochlorite) was diluted to 1.2% active chlorine and adjusted to pH'esof 5, 6, 7 and 8. The membranes were treated therein between 5 and 400sec. at room temperature, washed in tap water until no active chlorinecould be demonstrated in the wash water and the chlorine content of themembranes determined iodometrically. The chlorine contents as a functionof the treatment time are graphically shown (see FIG. 1).

It can be determined that the absorption of chlorine drops sharply withincreasing pH but continuously increases with rising treatment time. Itcan also be determined that chlorinated filters are clearly damagedmechanically at pH 8 whereas no damage was able to be determined at lowpH'es. Since the stability of hypochlorite solutions decreased due tothe development of elementary chlorine at low pH'es, pH 6 was selectedfor further chlorination tests.

The repetition of the test at pH 6 with reduced active chlorine content(0.12%) is shown in FIG. 2. These conditions were selected at achlorination time of 5 sec. as standard conditions for the graftingtests after Na hypochlorite chlorination.

EXAMPLE 2 Chlorination with t-butylhypochlorite

The membranes were treated 0-800 sec. in a 0.1% solution oft-butylhypochlorite in n-hexyne, methylene chloride or acetone, washedwith the particular pure solvent and the chlorine content determinedaccording to a photometric method using Aquaquant Cl (Merck). Thechlorine content as a function of the treatment time was graphicallyshown (see FIG. 3). It can be determined that the chlorine contentcontinues to constantly rise, after a strong increase in the firstseconds, with the treatment time and that there is a strong dependencyon the solvent used. Although the active chlorine content of thesolutions used is approximately 50% of that used in the second part ofexample 1, the chlorine absorption is below that of the referenceexample by at least a factor of 10.

It is assumed that the chlorine absorption, which is strong at first,can be traced to a chlorination of the surface which is followed by achlorination of the matrix interior.

EXAMPLE 3 Chlorination with chloramine T

The membranes were chlorinated at pH 6 for different times in a 0.5%aqueous solution of chloramine T (corresponding to an active chlorinecontent of 0.126%), washed and the chlorine content determined withAquaquant Cl. Graph see FIG. 4.

The chlorine content exhibits a much sharper rise at short chlorinationtimes than in the case of t-butylhypochlorite and remains largelyconstant after approximately 120 sec. It was determined in further tests(not shown), that practically the same final chlorine content isachieved at a chloramine T concentration of 10%.

This result is interpreted in such a manner that exclusively the surfaceis chlorinated with chloramine T. A slight further increase of thechlorine content is traced to slight amounts of inorganic hypochloritewhich are in equilibrium with chloramine T.

EXAMPLE 4 Grafting of HEMA after chlorination with Na hypochlorite

The membranes were chlorinated under the standard conditions cited inexample (corresponding to a chlorine content of 0.5%) and grafted forvarying times in a 10% solution of hydroxyethylmethacrylate in 0.1Mphosphate buffer pH 8 while dosing in Na dithionite solution formaintaining the redox potential constant. The redox potential was -340and -410 mV. The course of graftinq is shown graphically in FIG. 5, inwhich the degree of grafting represents the weight increase in % of theinitial weight.

A more rapid grafting takes place in the initial phase at higher redoxpotential, followed by a flattening. By way of contrast, the finaldegree of grafting after 20 h is higher at the lower redox potential.

FIG. 6 shows the flowthrough capacity of the grafted membranes incomparison to the degree of grafting. It turns out that at the samedegree of grafting, the decrease in flowthrough is greater at a lowerpotential. An increase of the flowthrough capacity can be observed againin both instances at degrees of grafting above 100-150%.

EXAMPLE 5 Grafting of HEMA after chlorination with chloramine T

The membranes were chlorinated 5 and 600 sec. in a 0.5% aqueous solutionof chloramine T at pH 6, corresponding to a chlorine content of 0.025and 0.05%. The grafting took place as described in example 4. The degreeof grafting as a function of the grafting time is shown in FIG. 7. Inthe initial phase the grafting took place more rapidly at the lowchlorine content. After approximately 90 min. the same degree ofgrafting is achieved in both tests. However, whereas the grafting is nowpractically concluded in the case of a low chlorine content, presumablybecause the chlorine content is consumed, the degree of graftingcontinues to rise in the case of a high chlorine content.

FIGS. 8 and 9 show the dependency of the surface growth and of theflowthrough rate on the degree of grafting. At the same degree ofgrafting, both the flowthrough rate and the surface growth for thelower-chlorinated specimens are above those of the highly chlorinatedspecimens. The latter thus preferably exhibit surface grafting.

EXAMPLE 6

Grafting of various monomers onto membranes chlorinated with Nahypochlorite according to the Ferongalite method.

Chlorination solution: Commercially available chlorine bleach liquor wasdiluted with RO water to 0.12% active chlorine and adjusted to pH 6 withdilute sulfuric acid.

Chlorination: the dry membranes were placed on the chlorination solutionand immersed for 2 sec., removed again, washed with RO water, washed 15min. in 1% solution of urea pH 10 and then washed 30 min. in RO water.

Grafting bath: Aqueous solution of 0.1% FeNaEDTA (Fe³⁺ -EDTA complex),0.1% EDTA, 1% rongalite, 2% Na carbonate, 10% monomer brought to pH 10with 2N sulfuric acid.

Grafting: The chlorinated before filter was grafted 10 min. at roomtemperature under slight agitation, washed 10 min. with RO water, washedtwice for 5 min. per time with acetone and dried 15 min. at 60 ° C. in acurrent of air.

    ______________________________________                                                         Degree of   Degree of                                                         grafting (%)                                                                              grafting (%)                                     Monomer          after 10 min.                                                                             after 30 min.                                    ______________________________________                                        Hydroxyethylmathacrylate                                                                       7.6         17.7                                             Methacrylamidoglycolate                                                                        2.3         3.8                                              methylether                                                                   Glycerol methacrylate                                                                          4.8         11.6                                             Sulfopropylmethacrylate                                                                        2.2         7.2                                              N-morpholinopropyl-                                                                            0.9         4.9                                              methacrylamide                                                                Acrylic acid     0.1         1.7                                              Vinyl pyrrolidone                                                                              2.6         7.1                                              N-acrylamidoglycolic acid                                                                      0.6         2.7                                              Methacrylamide   1.2         4.1                                              2-acrylamide-2-methyl-                                                                         1.1         3.2                                              propanesulfonic acid                                                          Methacrylamidopropyl-                                                                          0.7         1.6                                              trimethylammonium chloride                                                    ______________________________________                                    

EXAMPLE 7

Grafting of various monomers on membranes chlorinated withNa-hydrochlorite according to the Na dithionite method withoutadjustment of potential

Chlorination: Like example 6

Grafting bath: 0.1% Na dithionite in 0.15M phosphate buffer pH 8, 10%monomer, pH readjusted as required to 8.

Grafting: Like example 6

    ______________________________________                                                         Degree of   Degree of                                                         grafting (%)                                                                              grafting (%)                                     Monomer          after 10 min.                                                                             after 30 min.                                    ______________________________________                                        Diethyleneglycol-                                                                              30.1        70.3                                             methacrylate                                                                  Hydroxypropylmethacrylate                                                                      30.2        108.0                                            Hydroxyethylmethacrylate                                                                       54.3        155.0                                            Methacrylamidoglycolate-                                                                       34.6        88.1                                             methylether                                                                   Hydroxyethylacrylate                                                                           36.5        63.3                                             Hydroxypropylacrylate                                                                          19.4        54.5                                             Glycerol methacrylate                                                                          28.1        57.5                                             Acrylamide       37.2        37.9                                             Vinylacetate 12.9                                                                              29.2                                                         (saturated solution)                                                          Octaethylene glycol                                                                            8.0         20.6                                             methacrylate                                                                  Sulfopropylmethacrylate                                                                        8.1         10.2                                             N-morpholinopropyl-                                                                            8.1         10.2                                             methacrylamide                                                                Acrylic acid     1.8         2.9                                              Vinylpyrrolidone 15.7        15.2                                             Methacrylamide   2.8         3.2                                              2-acrylamido-2-methyl-                                                                         3.0         4.0                                              propanosulfonic acid                                                                           3.0         4.0                                              Vinylimidazol    3.2         3.1                                              Methylvinylacetamide                                                                           6.4         6.5                                              ______________________________________                                    

EXAMPLE 8

Continuous grafting of glycidylmethacrylate according to the emulsionmethod on a nylon 6 membrane web 40 cm wide, 30 m long (nominal poresize 0.45 μm) for further reactions to membrane ion exchangers and thelike

CAT solution: 2% solution of chloramine T adjusted with sulfuric acid topH 6.3.

Monomer emulsion: 6 kg glycidylmethacrylate are mixed with 120 gemulsifier (Arlatone G, ICI) and emulsified with 113.8 kg 0.1M Naphosphate buffer pH 8 for 5 min. with a blade agitator.

CAT solution and monomer emulsion were filled into treatment pans withdeflection rollers for the membrane web, at which time the monomer panis sealed to the atmosphere and charged with nitrogen. Two further pansare located between the CAT pan and the monomer pan of which the firstis filled with RO water which is replaced during the treatment with 1001/h and the second with phosphate buffer pH 8. The monomer pan isfollowed by a further wash pan with RO water, then comes the winding.The drying took place in a separate step on a drying drum of 70° C.

The monomer emulsion is brought to a redox potential of -310 mV bydosing in a solution of 3% Na dithionite and 3% sodium hydroxide (pH11-12) which potential is measured during the graftingpotentiometrically and maintained constant by means of redosing the 3%dithionite solution. The dwell time in the CAT bath is 2 min. the dwelltime in the monomer bath (grafting time) is adjusted via the web speed.Grafting time (min.) Degree of grafting (%)

    ______________________________________                                        Grafting time (min.)                                                                         Degree of grafting (%)                                         ______________________________________                                        5.7            15                                                             13             28.5                                                           ______________________________________                                    

EXAMPLE 9

Like example 8 but instead of the nylon 6 membrane, a nylon 6.6 fleecewith a surface weight of 30 g/m² is used. The degree of grafting is 7%at a grafting time of 15 min.

EXAMPLE 10 Examination of the oxidation resistance

Various membrane specimens with a nominal pore size of 0.2 μm weredeposited in the gas chamber of an autoclave of the design customarilyused for sterilization. The autoclave was heated to 142° C. but the airwas not removed by blowing off vapor, as is otherwise customary. Thisassured the simultaneous action of water vapor and atmospheric oxygen at142° C. After 1 h the membranes were removed and the bursting strengthtested. The measurement of bursting strength took place in such a mannerthat the moistened filters were loaded without support on a diameter of25 mm with an increasing gas pressure and the pressure at which thebursting of the membranes occurred was determined. All membranespecimens were non-reinforced. If a bursting strength was measurable,the treatment was multiply repeated. The result shows that the nylonmembranes of the market contains an extractable antioxidant and that thegrafted membrane exhibits the same oxidation resistance as the onestabilized with antioxidant.

    ______________________________________                                                        Number of  Bursting                                           Specimen        autoclavings                                                                             strength (bars)                                    ______________________________________                                        Nylon 6.6 non-grafted                                                                         0          0.30                                               Ultipor (Pall)  0          0.24                                                               2          0.25                                                               4          0.26                                               Ultipor (Pall), 0          0.24                                               extracted overnight                                                                           1          0.0                                                with ethanol                                                                  Nylon 6.6 grafted with                                                                        0          0.58                                               hydroxyethyl-   2          0.48                                               methacrylate    4          0.48                                               (degree of grafting 14%)                                                                      5          0.43                                               ______________________________________                                    

EXAMPLE 11 Examination of the γ radiation resistance

Various membrane specimens with a nominal pore size of 0.2 μm wereexposed to a γ radiation dose of 2.5 Mrads and the bursting strengththen determined as described in example 10.

    ______________________________________                                                         Burstng strength (bars)                                                         before    after                                            Specimen           irradiation                                                                             irradiation                                      ______________________________________                                        Nylon 6 membranes  0.28      0.01                                             Ultipor (Pall) without                                                                           0.22       0.015                                           antioxidant                                                                   Ultipor (Pall) with                                                                              0.23      0.02                                             antioxidant                                                                   Nylon 6 membrane according                                                                       0.29      0.06                                             to example 5 grafted with                                                     hydroxyethylmethacrylate                                                      (degree of grafting 10%)                                                      Nylon 6 mexbrane according                                                                       0.33      0.23                                             to example 8 grafted with                                                     glycidylmethacrylate                                                          (degree of grafting 28%)                                                      ______________________________________                                    

This example shows that polyamide membranes can be obtained by using thegrafting process of the invention which exhibit 70% of their originalmechanical strength after the action of the radiation dosage customaryin radiation sterilization.

EXAMPLE 12

Wettability of hydroxyethylmethacrylate-grafted membranes by liquidswith a high surface tension.

The time was determined for nylon 6 membranes with a nominal pore size0f 0.2 μm chlorinated according to example 5 after 600 sec. withchloramine T and grafted with hydroxyethylmethacrylate which timeelapses between the placing of 10 μl water (γ=72 dyn/cm) or solution ofcommon salt on the membrane surface and the complete adsorption of theliquid by the membrane (suction time). A 17% (γ=79 dyn/cm) and asaturated solution of common salt (26%, γ=82.6 dyn/cm) were used. Thenon-grafted membranes as well as commercial nylon 6.6 membranes wereused for comparison. The commercial product "Loprodyne" is a productwhich is surface-modified to elevate the hydrophilia. The tableindicataes the suction times in seconds.

    ______________________________________                                        Specimen     Water     17% NaCl  26% NaCl                                     ______________________________________                                        Non-grafted  33        >3600     no wetting                                   Degree of    15          200     no wetting                                   grafting 10%                                                                  Degree of    17          85      300                                          grafting 20%                                                                  Ultipor 0.2μ                                                                            21        >3600     no wetting                                   (Pall)                                                                        Loprodyne 0.2μ                                                                          20          210     no wetting                                   (Pall)                                                                        ______________________________________                                    

The wettability necessary for a special application can be adjustedwithin broad limits in the grafting process of the invention and allhydrophilic monomers can be used thereby. It is not necessary therebythat the monomer comprises hydroxyl groups. The ionic monomers can alsobe considered as well as neutral ones such as e.g. vinyl pyrrolidone.Glycidylmethacrylate, which does not bring about any hydrophilizingitself, can also be grafted for hydrophilic modification andsubsequently hydrolysed to glycerol methacrylate. The same applies tothe grafting of vinyl acetate and the subsequent saponification to vinylalcohol.

We claim:
 1. A process comprising surface grafting of microporouspolymers of nitrogen-containing polymers with ethylenically unsaturatedmonomers, by a process alternative to the application of high-energyradiation and/or the use of high temperatures, further by,a) removinghydrogen atoms on the nitrogen atoms of the polymer by halogen atomsusing inorganic N-halogen derivatives as halogenizing agent, b) removinga part of the halogen by reducing agents in the presence ofethylenically unsaturated monomers during a radical grafting of the sameon the nitrogen atoms and c) thereafter removing the remaining halogenatoms by reducing agents in the absence of monomers.
 2. The processaccording to claim 1, wherein a chlorinating agent is used ashalogenating agent.
 3. The process according to claim 2, wherein aninorganic hypochlorite, preferably Na hypochlorite in aqueous solutionis used as chlorinating agent.
 4. The process according to claim 3,wherein the pH of the chlorinating agent is 5 to 7, preferablyapproximately
 6. 5. The process according to any one of claims 3 and 4,wherein the active chlorine content of the chlorinating agent is 0.05 to3%, preferably 0.1 to 0.5%.
 6. The process according to any one ofclaims 3 to 5, wherein the chlorination is carried out at roomtemperature and the chlorination time for effecting a matrix grafting is30 s. to 5 min., preferably 1 to 2 min.
 7. The process according to anyone of claims 5 to 6, wherein the chlorination is carried out at roomtemperature and the chlorination time for effecting a pure surfacegrafting is 5 to 15 min., preferably approximately 10 min.
 8. Theprocess according to claim 2, wherein an organic hypochlorite,preferably t-butylhypochlorite, dissolved in an inert organic solvent isused as chlorinating agent.
 9. The process according to claim 8, whereinn-hexane, methylene chloride or acetone is used as organic solvent. 10.The process according to claim 2, wherein the sodium salt ofN-chloro-p-toluene sulfonamide with a formula weight of 230 and ahydrolysis constant of approximately 10⁻⁸ is used as chlorinating agent.11. The process according to any one of claims 10 and 14, wherein theconcentration of the chlorination of the chlorinating agent is 0.1 to3%, preferably 1 to 2%.
 12. The process according to any one of claims10 to 11, wherein chlorination times of 2 to 10 s. are used forachieving a matrix grafting.
 13. The process according to any one ofclaims 10 to 11, wherein chlorination times of 0.5 to 10 min.,preferably of 1 to 5 min. are used for achieving a pure surfacegrafting.
 14. The process according to claim 2, wherein the pH of thechlorinating agent is between 5 and 7, preferably approximately 6.3. 15.The process according to claim 2, wherein dichloroisocyanuric acid isused as chlorinating agent.
 16. The process according to claim 1,wherein an inorganic hypobromite is used as halogenizing agent.
 17. Theprocess according to claim 16, wherein the inorganic hypobromite isproduced in situ by exposing the polymer to be brominated successivelyto an atmosphere of bromine and one of ammonia.
 18. The processaccording to any one of claims 1 to 17, wherein mono unsaturatedmonomers are used as ethylenically unsaturated monomers.
 19. The processaccording to claim 18, wherein acrylic acid, methacrylic acid and itsesters or amides are used as ethylenically unsaturated monomers.
 20. Theprocess according to claim 18, wherein mono ethylenically unsaturatedmonomers containing hydroxy groups are used.
 21. The process accordingto claim 20, wherein the ethylenically unsaturated monomer is selectedfrom the group consisting of:HydroxyethylmethacrylateMethacrylamidoglycolate methylether Glycerol methacrylateSulfopropylmethacrylate N-morpholinopropylmethacrylamide Acrylic acidVinyl pyrrolidone N-acrylamidoglycolic acid Methacrylamide2-acrylamide-2-methyl-propanesulfonic acidMethacrylamidopropyltrimethylammonium chlorideDiethyleneglycolmethacrylate HydroxypropylmethacrylateMethacrylamidoglycolate methylether HydroxyethylacrylateHydroxypropylacrylate Acrylamide Vinylacetate Octaethylene glycolmethacrylate Vinylimidazol Methylvinylacetamide andGlycidylmethacrylate.
 22. The process according to any one of claims 1to 17, wherein polyunsaturated monomers are used as ethylenicallyunsaturated monomers.
 23. The process according to claim 22, wherein thepolyunsaturated monomer is selected from the group consisting ofpentaerythrite dimethacrylate, glycerol dimethacrylate, tetraethyleneglycol dimethacrylate and tetraethylene glycol diacrylate.
 24. Theprocess according to any one of claims 1 to 23, wherein mixtures ofmonomers are used.
 25. The process according to any one of claims 1 to24, wherein the grafted, ethylenically unsaturated monomers arecross-linked.
 26. The process according to claim 25, wherein thecross-linking is carried out using bifunctional monomers simultaneouslywith the grafting.
 27. The process according to any one of claims 1 to26, wherein the monomers are added to the grafting solution in the formof an emulsion.
 28. The process according to any one of claims 1 to 27,wherein the reducing agent is selected from the group consisting ofsodium dithionite, rongalite, hydrazine and ascorbic acid to remove thehalogen atoms during the grafting.
 29. The process according to claim28, wherein ascorbic acid is used in the alkalinge range.
 30. Theprocess according to claim 28, wherein rongalite is used at temperaturesabove 40° C.
 31. The process according to claim 28, wherein rongalite isused in combination with EDTA-complexed Fe²⁺ at a pH of approximately10.
 32. The process according to claim 28, wherein sodium dithionite isused as reducing agent at a pH of 6 to 10 preferably 7.5 to 8 and in aconcentration of 0.02 to 1%.
 33. The process according to claim 32,wherein sodium dithionite is used at a redox potential of -100 to -700mV, preferably between -100 and -500 mV.
 34. The process according toclaim 33, wherein the redox potential is potentiometrically determinedduring the grafting and maintained constant by dosing in a sodiumdithionite concentrate.
 35. The process according to any one of claims 1to 34, wherein the nitrogen-containing polymer is selected from thegroup consisting of aliphatic polyamides, polyurethanes,polysulfonamides and of polymers which do not have the nitrogen in themain chain but in the side chain.
 36. The process according to claim 35,wherein the polymer is in the form of foils or capillaries.
 37. Theprocess according to claim 35, wherein the polymer is in the form offibers having a thickness range of 1 to 100 μm and a surface/mass ratioof 0.4 to 4 m² /g.
 38. The process according to claim 35, wherein thepolymer is in the shape of fibers or of tissues or fleeces producedtherefrom.
 39. The process according to claim 35, wherein the polymer isin the shape of a membrane.
 40. The process according to any one ofclaims 1 to 39, wherein areal web materials are used for grafting. 41.The process according to claim 40, wherein the process is carried out ina discontinuous manner on a jigger.
 42. The process according to claim40, wherein the process is carried out in a continuous manner in agrafting bath.
 43. The process according to claim 42, wherein themonomer concentration is maintained constant by using a monomer oflimited water solubility and by maintaining the saturationconcentration.
 44. The process according to claim 43, wherein thesaturation concentration of the monomer is reduced by means of theaddition of electrolytes.
 45. The process according to claim 40, whereina web is impregnated with the grafting solution and that the grafting iscarried out in a continuous manner in a dwell time under inert gas.